In an effort to identify genetic differences that make some strep bacteria more virulent than others, scientists recently compared the genomes of more than 250 bacteria that had been collected from patients in Ontario over the course of a decade.

Many of the most virulent strains in the study, they found, carried genes from viruses, including some genes for toxins that harm humans. The genes from the viruses may account for some of the increased strength of the bacteria. Some of the genes apparently had spread among the bacteria, as can happen when bacteria come in contact with one another.

The researchers studied a type of Group A Streptococcus (GAS) bacteria known as “M3,” which is among the most harmful types of GAS bacteria. They can cause a range of illnesses, from strep throat and impetigo to rheumatic fever, toxic shock, and “flesh-eating” disease, also known as necrotizing fasciitis.

Based on the findings, which appear online in the Proceedings of the National Academies of Science, the researchers are revising their long-held views of the M3 strains.

“Before this study, we had lumped serotype M3 strains into a single category,” says James M. Musser of the National Institute of Allergy and Infectious Diseases in Rockville, Maryland. “Now we know that there are a large number of subtypes and which ones were able to be successful and cause disease.”

Musser and his colleagues sequenced the genome of an M3 strain that is representative of the pathogens that caused the strep epidemics in Ontario in 1995 and 2000. The sequencing was done at Baylor College of Medicine, in Houston, Texas.

Armed with the sequence data, the scientists compared the reference genome to those of bacteria from patients collected over an 11-year period by Don Low, a microbiologist at Mt. Sinai Hospital in Toronto. The comparison revealed evidence of genes from viruses in the strep bacteria that had been isolated during the epidemics.

In Ontario, which has about 11 million people, about 40 people fall ill each year with necrotizing fasciitis, and another 40 develop streptococcal toxic shock. The Centers for Disease Control and Prevention in Atlanta reports that there were 9,000 severe cases of GAS reported in the United States in 2002.

James M. Musser et al. Genome-wide molecular dissection of serotype M3 group A Streptococcus strains causing two epidemics of invasive infections. Published online in Proceedings of the National Academy of Sciences, July 26, 2004.